The present invention relates generally to methods for determining the location of a transitional boundary or xe2x80x9cshadowlinexe2x80x9d between an illuminated region and a dark region on a linear scanned array of photosensitive cells, such methods being particularly applicable to critical angle refractometers wherein shadowline location is correlated to index of refraction of a test sample.
Refractometers are widely used for measuring the refractive index of a sample. In refractometers designed to measure solid and/or liquid samples, the critical angle of total reflection is measured by directing an obliquely incident convergent beam of light at a surface-to-surface boundary between a high refractive index prism and the sample and then observing a portion of the light after interaction at the boundary. In transmitted light refractometers, light that is transmitted through the sample and prism is observed, while in reflected light refractometers, the light that is reflected due to total reflection at the surface-to-surface boundary is observed. In either case, an illuminated region is produced over a portion of a detection field of view, and the location of the shadowline between the illuminated region and an adjacent dark region in the detection field of view allows the sample refractive index to be deduced geometrically. In simpler hand-held refractometers used in industry, a reticle scale is superimposed in the field of view and the user looks through an eyepiece to observe the location of the shadowline with respect to the reticle scale, which is marked so as to provide desired information such as percentage concentration of solids in the sample.
Automatic refractometers were developed to remove the guesswork associated with visually determining shadowline location with respect to a reticle scale, thus improving the accuracy (closeness to the true value) and precision (repeatability regardless of accuracy) of measurement readings. U.S. Pat. No. 4,640,616 issued Feb. 3, 1987 to Michalik discloses an automatic Abbe refractometer wherein a linear scanned array (LSA) of photosensitive elements or xe2x80x9ccellsxe2x80x9d is arranged to detect light totally reflected at a sample/prism boundary. In a commercial embodiment, the linear scanned array includes a straight line of charge-coupled device (CCD) cells that are scanned electronically to provide a series of pulse signals each having an amplitude proportional to the amount of illumination received by the cell from incident light. Light received by the linear scanned array divides the array into an illuminated region and an adjacent dark region, thereby forming a shadowline on the array. The particular cell or interpolated inter-cell fraction at which the shadowline falls on the linear scanned array, known as the xe2x80x9ccell crossing number,xe2x80x9d is determined by the index of refraction of the sample substance placed in contact with the optical means. Thus, a method is required for evaluating the pulse signals from the photosensitive cells to find the cell crossing number. The cell crossing number may then be used to provide a measurement value of the index of refraction or a percent concentration of dissolved solids, such as sucrose, in the sample medium.
The Michalik patent teaches a xe2x80x9cthresholdingxe2x80x9d approach for processing the light intensity signals from the array cells to determine the cell crossing number. This approach is represented graphically at FIG. 3 herein. Under a thresholding approach, an empty baseline or reference scan is taken without a sample (that is, with respect to air) and stored to establish a reference illumination curve. The curve from the reference scan is then scaled by a predetermined fixed scale factor, for example 94%, to provide a threshold curve as indicated in FIG. 4. The resulting cell where the sample scan curve intersects the threshold curve is declared the cell crossing number.
This approach yields precise and accurate measurements, however the incident light levels must be controlled to a high degree, as it is crucial that the reference and sample scans are comparable. Any deviation in intensity levels will cause erroneous readings. Since the disclosed automatic refractometer is a xe2x80x9creflected lightxe2x80x9d refractometer, light reaching the linear scanned array never passes through the sample, it is possible to adequately control incident light levels by controlling source luminance. An advantage of the thresholding approach is that a fresh reference scan is taken every time the instrument is turned on, so gradual changes in the response characteristics of each photosensitive cell over the lifetime of the cell (known as xe2x80x9cresponse driftxe2x80x9d) do not affect instrument performance. Moreover, use of a reference scan compensates for cell-to-cell variance in response to a given level of illumination.
The use of a reference scan and scaled threshold is problematic in xe2x80x9ctransmitted lightxe2x80x9d refractometers because, unlike the reflected light refractometer discussed above, light must pass through the sample before reaching the detector array. Consequently, sample-dependent factors such as the color, opaqueness, thickness, and homogeneity of the sample make it impractical to control incident light levels at the detector array. For example, a reference scan of air may be suitable for a clear water sample, but would not be suitable for measurement of a sample with low transmissivity, such as ketchup. Another drawback of the threshold approach is that a single defective cell providing an erroneous response signal may, at the worst, distort the measurement result.
U.S. Pat. No. 5,617,201 issued Apr. 1, 1997 to Kxc3xa5hre describes a reflected light refractometer using another method for determining the cell crossing number of the shadowline boundary. The method involves describing the illumination distribution curve by means of a mathematical model, and using the mathematical model to find the cell crossing number. In the described embodiment, the illumination distribution curve is represented by three different straight lines A, B, and C representing a light region of the array, a transition region of the array from light to dark, and a dark region of the array, respectively. The intersection of line B with line C is chosen as the cell crossing number. Non-linear models are also suggested. A similar approach with respect to a transmitted light refractometer is taught in U.S. Pat. No. 6,172,746 issued Jan. 9, 2001 to Byrne et al. (this patent shares a common assignee with the present application), and is illustrated herein at FIG. 5. However, the method involving intersecting xe2x80x9cbest fitxe2x80x9d straight lines representing the transitional and dark regions of the detector array proved to be too imprecise for transmitted light applications wherein detected light levels are difficult to control. The precision attained was inadequate over the range of light intensities that this instrument can experience, and thus the method was ultimately not adopted.
Therefore, it is a primary object of the present invention to provide a new method for determining the cell crossing number of a shadowline on a photosensitive array that is relatively immune to light intensity level variations while maintaining adequate precision.
This object is achieved by a method that is now briefly described in a preferred form. Initially, the photosensitive array is scanned to extract a response signal from each of the photosensitive cells in the array that represents the amount of illumination of the corresponding cell by incident light. The response signals from the photosensitive cells are converted from analog form to digital pixels, thus yielding a set of data points that collectively represent an illumination distribution curve over the array. A range of cells within which the shadowline resides is established by analyzing the illumination curve data. A preferred procedure for establishing a xe2x80x9cstartxe2x80x9d cell for this range is to find the brightest cell by looking for a peak pixel value, and then step forward one cell at a time until a cell having a pixel value that is 25% of the peak pixel value is reached. A preferred procedure for establishing an xe2x80x9cendxe2x80x9d cell is to begin at the xe2x80x9cstartxe2x80x9d cell and step forward one cell at a time, continually updating the lowest pixel value that is read, until a cell that has a pixel value of 105% of the lowest pixel value is found or the last cell of the array is reached. The second derivative of the illumination distribution curve over the established range of cells is calculated and the greatest positive area bounded by the second derivative is identified. Finally, the centroid of the greatest positive area is found and its cell number coordinate is deemed the cell crossing number of the shadowline.
The above method is employed in an automatic refractometer for measuring refractive index of a sample substance. The refractometer comprises a linear scanned array having a plurality of photosensitive cells and optical means for directing light onto the array, wherein the particular photosensitive cells illuminated by the light and a cell crossing number of a shadowline defined by illuminated and dark regions of the array are determined by the index of refraction of a substance placed in operative association with the optical means. The refractometer further comprises analog to digital conversion means and digital processing circuit means for carrying out the aforementioned method, and an output device for reporting a measurement result derived from the cell crossing number.